If you look across the Bible, there are many instances where oil is used. The anointing of Saul and David; Jacob pouring oil over the rock where he had his dream of the staircase to heaven(?); Maria pouring oil on Jesus, etc. Was oil expensive back then or something?
The short answer is yes, oil was expensive. It was used in ceremonies both because it was often perfumed to have a pleasing aroma and because the financial sacrifice was an act of humility.
You mentioned several good examples, but I figured I would offer you one more: Elisha providing for a widow (2 Kings 4:1-7). In this scenario, Elisha is able to provide the woman with enough oil that she can pay off all her late husband's debts, and she and her sons are able to live on the rest.
Now, this passage doesn't explicitly state that oil was expensive, but a little common sense will tell us it can't have been cheap. Elisha sends the woman's sons for
empty vessels, and not too few, so he was clearly preparing to give her a large volume of oil, but it's fair to guess that the number ended up being in the 20-50 range (obviously just an estimate) as opposed to in the hundreds or thousands. For that amount of oil to be enough to pay off debts and set multiple people up for life means that oil had value, far more than it does today (imagine how much olive oil you'd need to hoard to be set for life).
Oil is a healing substance rubbing it all over you will literally strengthen your skin and thereby giving you strength to carry on. That's one of the reasons oil is an excellent symbol for the Holy Spirit and furthermore why it's used all over the Old Testament.
Any time you read oil, wind, water or fire you can read "Holy Spirit" unless otherwise indicated.
I know that in a historical/economical sense, this is not an answer to your question. But if you wanted a non-doctrine answer you probably should ask on BH.SE. According to Catholic Doctrine, blessed oil is a sacramental; and in this case it is the means by which actual graces can be conferred and we have a few distinct oils that are used for this. One for Anointing of the Sick; another for consecrating priests and altars; and another for baptizing and confirmation.
So, following biblical traditions and ancient cultural practices, oil has always been used in this way. As far as the woman perfuming Jesus' feet and the gifts of the Magi, the value of their oils were in the substances mingled with the oils. Oil itself would have had to been a fairly common substance since it was used to make bread (ala 1 Kings:17).
I've always thought of it as more of an issue of versatility and approachability. Oils can be used for everything. The only thing on earth which is more useful is water (unless you're willing to start debating gasses &c., in which case I'm going with Hydrogen and Oxygen for the win). But, unlike water, which will often suppress fragrance and flavor, oil will absorb and augment it. Smell a child after a Catholic baptism or confirmation (and I know there are other denominations who do the same, but I am looking only for an example). In addition to that, it also has use as a material to light the home (no symbolism there).
Oil, if it is properly stored, also has a remarkably long shelf-life. Look at the expiration date on your next purchase of olive oil. It won't be any time soon. Oil also has medicinal use.
So, we have oil associated with light and with food, it lasts a long time, it can heal, it is something which can be fragrant, and it can be gotten relatively easily (except in times of drought) as it is one of the major uses of olives (something not exactly uncommon in that area). Is there any wonder that it is significant?